Why does my husband accuse me of cheating, and I'm not, but he has cheated, and I forgiven him.

Because he is paranoid, and thinks you are as stupid, and immoral as he is. Simply put, that's his guilt freaking out his mind.

It sounds like he's projecting his own insecurities onto you. He cheated on you, and he can't imagine why you wouldn't cheat on him to get him back. It doesn't sound like a healthy situation. I would either seek some couples counseling or look into what he's doing. He may me cheating again.

He's cheating.

Because if he conveniences himself that you are cheating then he has given himself excuse to cheat himself.